Light-induced absorbance changes have been measured in the phototactic slime mold, Dictyostelium discoideum, and the phototropic fungus, Phycomyces blakesleeanus, which appear to be due to the photoreceptor pigments in these photoresponsive organisms. These light-induced absorbance changes have been observed in the mitochondria fraction of a cell-free homogenate of Dictyostelium cells and in the soluble supernatant obtained from sonicated mitochondria. The pigment responsible for these light-induced absorbance changes will be purified. Similar procedures will be attempted to isolate, purify and identify the photoresponsive pigment from Phycomyces. A photostimulation of respiration also occurs in Dictyostelium cells which may underlie the phototactic response. Action spectra for the light-induced absorbance change, the photostimulation of respiration, and the phototactic response of the organism will be compared. The photostimulation of respiration will be examined in mitochondria and in soluble extracts in an effort to purify and identify the photoreceptor pigment.